Bacteria have long been hard at work cleaning up uranium-contaminated …

Bacteria have long been fighting on the front lines of uranium-contaminated groundwater. Their ability to take uranium out of a solution and mineralize it has proven invaluable at abandoned uranium mines. The mechanism by which they accomplish this fortunate feat has remained a mystery—until now. A paper in the Proceedings of the National Academy of Sciences this week reveals the details.

Bacteria of the genus Geobacter can help remediate a number of different groundwater contaminants. Besides uranium, they can also take on petroleum compounds and chlorinated solvents—two of the most widespread contaminants. They are anaerobic bacteria, meaning that they live in the absence of dissolved oxygen. Instead, they utilize a variety of elements that includes nitrogen, manganese, iron, sulfur, and, yes, uranium.

It comes down to chemical oxidation/reduction reactions, which are basically electron transfers. The things the bacteria are munching on—petroleum compounds, chlorinated solvents, and other organic compounds—are electron donors. The elements used for respiration (i.e. oxygen, iron, etc.) are electron acceptors. Energy is released to the cell as part of this reaction, which is, of course, why living things go through all the trouble to do it.

It’s well known that some members of Geobacter can utilize uranium as an electron acceptor, similar to how humans use oxygen. As part of that electron transfer reaction, hexavalent uranium (which has a charge of +6) becomes tetravalent uranium (which has a charge of +4). Hexavalent uranium is easily dissolved in water, meaning it is mobile in the subsurface and moves with groundwater flow. Tetravalent uranium, however, quickly precipitates in mineral form, locking it up in the sediment or bedrock. This is what makes the bacteria so helpful at uranium contamination sites—they prevent contamination from spreading in groundwater.

So, do these bacteria produce an enzyme that helps them process uranium in this way, or is there something else going on? That’s been the question for some time now. A group of microbiologists suspected that pili—tiny, thread-like appendages on the surface of many bacteria—might have something to do with it. To test this, they compared mutants that could not develop pili to normal bacteria. They found that the pili were indeed central to the mineralization of uranium. To understand why, we’ve got to include a study that was published in Nature Nanotechnology just last month.

The second study discovered that the pili of that same Geobacter species are electrically conductive. In fact, they are just as conductive as metallic nanowires. As if the first example of metal-like conductivity in a biological structure wasn’t enough, the researchers were even able to get films of these bacteria to function as transistors.

It makes sense, then, that the pili would act as electron conduits to facilitate oxidation/reduction reactions. But why go to such lengths when the uranium could just be brought into the cell? There are a couple reasons. First, the long pili greatly increase the bacteria’s surface area, making more of it available for reactions. Second, it protects the cell. Mineralization of the uranium inside the cell could really mess with its vital functions. The pili-deficient mutants were not only less effective at utilizing (and mineralizing) uranium, their respiratory activity also slowed as they took in ever more uranium to accept electrons.

So what’s the practical upshot of this knowledge? If we understand how these bacteria go about their work of attenuating groundwater contamination, we’re better equipped to facilitate the conditions required to optimize uranium mineralization. What’s more, we may even work out how to engineer synthetic or biological improvements to the process.

I worked for Flow General Corp in the chemotherapeutic agent repository way back in the eighties and I logged chemical compounds from a bunch of feeder organizations into a storage center that was used by the National Cancer Institute for screening cancer treatments. I was responsible for (manually) creating sample vial labels so I had to read all of the material information off the manifests. We had one from Frederick Cancer Research Center (formerly Fort Detrick Biological Warfare unit) which had a description that included "causes e. coli to excrete plutonium" which most of us thought was a joke at the time....but sounds like it might have been true?

How can these biological structures have conductivities similar to what you see from metallic nano structures where the current is carried by free electrons? As far as I know biological conductance over larger distances is usually based on ion transport mechanisms (For example conductivity in nerves, proton gradients, etc.) Do these pili use electron transport chains to move the electrons along the pili?

I think the title was meant to be a joke. This is a story about Multiple Oxidizing Organelles; so uranium-MOOching ought to be an obvious enough title which is why muching might have been funny word play you would easily flag if you had ever smelt a scent of humors. You can easily buy humors anywhere you can buy internet enabled irony detectors.

How can these biological structures have conductivities similar to what you see from metallic nano structures where the current is carried by free electrons? As far as I know biological conductance over larger distances is usually based on ion transport mechanisms (For example conductivity in nerves, proton gradients, etc.) Do these pili use electron transport chains to move the electrons along the pili?

My girl friend is actually the third author on this paper. Her lab believes that the pilin protein which is the structural component of the pili can carry current independent of cytochromes (though I know some researchers believe that there must be cytochromes tethered to the pili by some wobbly proteins that let them bump into each other to exchange electrons.

How can these biological structures have conductivities similar to what you see from metallic nano structures where the current is carried by free electrons? As far as I know biological conductance over larger distances is usually based on ion transport mechanisms (For example conductivity in nerves, proton gradients, etc.) Do these pili use electron transport chains to move the electrons along the pili?

My girl friend is actually the third author on this paper. Her lab believes that the pilin protein which is the structural component of the pili can carry current independent of cytochromes (though I know some researchers believe that there must be cytochromes tethered to the pili by some wobbly proteins that let them bump into each other to exchange electrons.

I think the title was meant to be a joke. This is a story about Multiple Oxidizing Organelles; so uranium-MOOching ought to be an obvious enough title which is why muching might have been funny word play you would easily flag if you had ever smelt a scent of humors. You can easily buy humors anywhere you can buy internet enabled irony detectors.

It's one thing to be rude to people for asking about an inside joke they didn't get, but a whole nother thing to explain one to others so that you can then be rude. Nerdy bullies are so cute.

Yeah, except that our motivations, which are prior to us doing anything, includingthinking about our motivations, come to us out of the blue, unpredictably. They cannotbe known before they arise within us which means we are not creating them. Theyare "creating" us. So forget about free will, it doesn't exist. In fact, forget about anykind of control, we don't have any. We bob when the bobbing agent hits, only to think we are the bobbers in charge. We aren't, so gods we ain't. And what'sgod anyway? And how would he/she/it know? Who would have been there to tell godhe/she/it was god?

Thank you, just wondering if this could have been used in the gulf and at some of the chemical plants.

The microbes that do live in the Gulf and break down petroleum products are different, but I don't know the details of their biology. Oftentimes in bioremediation, it's not about bringing in the bacteria as much as it is about stimulating those that are naturally present, making sure they have the right conditions and nutrients, etc. That's one thing when you're cleaning up a plume in groundwater in an old industrial park-- it's another thing working in the open ocean.

At any rate, I know very little about the Gulf bugs, so I'm afraid I can't give you a good answer.